Syndiotactic polypropylene has high transparency and surface gloss and also excellent flexibility as compared with conventional isotactic polypropylene. Therefore, it is desired that the syndiotactic propylene is used for new uses in which conventional isotactic polypropylene cannot be used, in addition to uses such as films, sheets, fibers, injection molded products and blow molded products that are known to be the uses of conventional isotactic polypropylene.
It is conventionally known that syndiotactic polypropylene can be produced by a method of polymerization at a low temperature using a catalyst made from a vanadium compound and an organoaluminum compound. In the conventional method, however, the obtained polymers do not exhibit sufficient properties because the polymer does not have sufficient high syndiotacticity and sufficient high melting point.
Under the circumstances, a method for producing syndiotactic polypropylene using a polymerization catalyst comprising a metallocene compound in order to enhance syndiotacticity has been studied hard. For example, J. A. Ewen and others disclose a method of polymerizing propylene syndiospecifically using a catalyst comprising isopropyl(cyclopentadienyl-1-fluorenyl)hafnium dichloride and aluminoxane (Non-patent document 1). JP-A-H4 (1992)-80214 (Patent document 1) discloses that polypropylene having high syndiotacticity can be produced using, as a catalyst, a metallocene compound with a specific structure having two transition metal atoms in the molecule. JP-A-H5 (1993)-209019 (Patent document 2) discloses a method that a syndiotactic propylene copolymer is produced using a catalyst comprising a specific aluminoxane and a metallocene compound typified by [phenyl(methyl)methylene](9-fluorenyl) (cyclopentadienyl)hafnium dichloride, at a temperature of from −40 to 150° C. in a solution, in a suspension or in a gas phase. JP-A-H8 (1996)-67713 (Patent document 3) discloses that a method of producing syndiotactic polypropylene using, as a co-catalyst, a borate and a metallocene compound such as rac-2,2-dimethyldimethylpropylidene(1-η5-cyclopentadienyl)(1-η5-fluorenyl)dichlorozirconium, in a diluted hydrocarbon suspension. WO01/19877 (Patent document 4) discloses a method of producing syndiotactic polypropylene using a polymerization catalyst comprising a metallocene compound having a specific structure and methyl aluminoxane in the pamphlet. Moreover, JP-A-2002-510358 (Patent document 5) discloses a method of producing syndiotactic polypropylene by propylene bulk polymerization using, as a catalyst, isopropylidene (cyclopentadienyl) (3,6-di-tert-butylfluorenyl) zirconium dichloride and methyl aluminoxane in Example 11.
Any of the above known techniques does not disclose a process for producing syndiotactic polypropylene having high stereoregularity (syndiotacticity) and high melting point by solution polymerization in an industrial scale.
Patent Document 1: JP-A-H4 (1992)-080214
Patent Document 2: JP-A-H5 (1993)-209019
Patent Document 3: JP-A-H8 (1996)-067713
Patent Document 4: Pamphlet of WO01/019877
Patent Document 5: JP-A-2002-510358
Non-Patent Document 1: J. Am. Chem. Soc. 1988, 110, 6255